1. Field of the Invention
The invention relates to a four-wheel-drive vehicle, particularly to a four-wheel drive articulate working vehicle such as a riding lawn mower. More particularly, it relates to a structure of a transaxle apparatus in the vehicle, and a structure for drivingly connecting front and rear transaxle apparatuses in the vehicle to each other.
2. Related Art
Conventionally, there is a well-known articulate riding lawn mower with pivotally connected first and second frames so as to allow the second frame to be folded relative to the first frame around a vertical axial pivot by steering operation (i.e., manipulation of a steering wheel). The first frame is equipped with an engine and a transaxle apparatus supporting a first axle driven by the engine. The second frame is equipped with a working device such as a mover device, a driver's unit, and an axle casing supporting a freely rotatable second axle. Furthermore, a riding lawn mower provided with an Ackerman type steering system is also well known.
For example, the Japanese Patent Laid Open Gazette 2000-270,651 discloses an articulate four-wheeled lawn mower in which the first frame serves as a rear frame and the second frame serves as a front frame. The rear frame is provided with an HST (a hydrostatic transmission) for transmitting engine power to rear wheels supported by the rear frame. Also, the rear frame is provided with a power take-off shaft for taking out power from a pump shaft of a hydraulic pump of the HST so as to transmit the rotation of the power take-off shaft to a mower device supported by the front frame.
Furthermore, as well known from Japanese Laid Open Gazette Sho. 63-42616, for example, some articulate vehicles have such a feature that the first and second frames differ in length and the first and second axles are unequally distant from the pivot in correspondence of the length difference between the frames.
Each of the above-mentioned conventional vehicles is a two-wheel drive vehicle, in which the second axle supported by the axle casing on the second frame (usually serving as a front frame) rotates freely from the engine power driving the first axle supported by the transaxle apparatus of the first frame (usually serving as a rear frame).
The two-wheel-drive vehicle driving only rear wheels is advantageous in steering performance, however, it lacks stability when working on a slope and road ability when running on a bad road. Moreover, the vehicle mired in mud or the like is difficult to bail out.
As a conceivable manner for solving the problem, a four-wheel drive articulate vehicle may be provided. However, in the above-disclosed vehicle including the rear frame on which the engine, the HST and the power take-off shaft are mounted, the power take-off shaft is rotated synchronously to rotation of the pump shaft so that the rotary speed of the power take-off shaft is constant as long as the rotary speed of the engine is constant, while the rotary speed of the rear wheels driven by output of the hydraulic motor is changed by speed change operation for adjusting a swash plate angle of the hydraulic pump. Thus, the power take-off shaft for driving the working device cannot be used as a front-wheel drive shaft simply. Even if another power take-off shaft is allowed to be provided in the transaxle apparatus on the rear frame so as to take out output power of the HST and to drive the front wheels synchronously to the rear wheels, there is considerable limitation in arrangement of a mechanical transmission system between the transaxle apparatuses on the front and rear frames because it is necessary to ensure the rotatability of both the frames and to arrange the transmission system for driving the working device in the place. The four-wheel drive articulate vehicle disclosed by U.S. Pat. No. 6,425,452, in which both the transaxle apparatuses on the respective first and second frames incorporate respective HSTs, is very expensive and requires both the HSTs to be controlled synchronously.
Further, the front and rear axles of the above-disclosed four-wheel-drive articulate working vehicle of the US Patent '452 are equally distant from the pivot between the front and rear frames so that the vehicle may turn smoothly without dragging any of front and rear wheels by keeping a constant relative velocity between the first and second axles whether the vehicle goes straight or turns.
However, if the above-disclosed articulate working vehicle of the Japanese Document '616 having the front and rear axles being unequally distant from the pivot between the front and rear frames is simply made into a four-wheel drive vehicle, a ratio of a distance between one axle and a turning circle center of the vehicle to a distance of the other axle from the center varies along with variation of steering operation degree (the steering wheel angle). Therefore, a constant relative velocity between the front and rear axles kept whether the vehicle travels straight or turns causes that either front wheels or rear wheels are dragged so as to prevent the vehicle from smooth turning.
Consequently, the four-wheel-drive articulate working vehicle with the first and second axles being equally distant from the pivot between the first and second frames can not turn smoothly unless the relative velocity between the first and second axles, the steering degree (the steering wheel angle), the turning angle of the vehicle (the angle difference between the first and second axles), and the turning circle center of the vehicle are well associated.